1. Field of the Invention
The present invention relates to semiconductor light-emitting devices (including light-emitting diode (LED) and laser diode (LD)) and particularly to improvements in operating voltage, luminous efficiency, lifetime, and yield of nitride-based semiconductor light-emitting devices.
2. Description of the Background Art
Nitride semiconductor materials such as GaN, InN, AlN, and mixed crystals thereof have a band gap where direct interband transition occurs. In particular, a mixed crystal of InGaN can emit radiation in the wavelength range from red to ultraviolet, and accordingly attracts attention as a material for short-wavelength radiation. A light-emitting diode capable of emitting radiation in the wavelength range from ultraviolet to green has already got practicability by utilizing the mixed crystal of InGaN. Further, a bluish violet laser diode achieves a lifetime longer than 10,000 hours under a condition of continuous lasing at the room temperature. As such, semiconductor light-emitting devices for short-wavelength radiation are making rapid progress toward commercialization thereof.
One of factors for such rapid progress is that ELOG (Epitaxial Lateral Over Growth) technique can reduce the dislocation density in a nitride-based semiconductor layer. That is, it has been found in recent years that application of ELOG technique to growth of a GaN layer on a sapphire substrate is effective in reduction of dislocations which are generated when the GaN layer is grown by HVPE (Hydride Vapor Phase Epitaxy) method. The GaN layer grown by ELOG technique includes less defects of threading dislocations and the like. It is accordingly reported that an LD produced by using such a GaN layer can exhibit a longer lifetime. On the other hand, it is proposed to use a thick film of GaN produced by HVPE as a substrate. The substrate of such a thick GaN film can be used to reduce crystal defects in a nitride-based semiconductor layer grown on the substrate by metal-organic chemical vapor deposition (MOCVD) etc., promising a longer lifetime of a resultant nitride-based semiconductor light-emitting device.
Although currently produced GaN-based substrates include dislocation defects reduced to some degree by utilizing the ELOG technique etc., they still have a considerably higher dislocation density than that of other group III-V compound semiconductor substrates such as GaAs substrate. Moreover, N and Ga are likely to escape out of the GaN substrate, especially out of the substrate interface, due to a high equilibrium vapor pressure of nitrogen, which causes an increased defect density. Therefore, a nitride-based semiconductor light-emitting device formed by MOCVD on a GaN substrate still contains lots of crystal defects. Such defects act as centers for non-radiative recombination, and the defective portions serve as current paths to cause current leakage. Here, a problem is that light-emitting devices containing lots of crystal defects need higher drive voltage and result in less yield.
In particular, crystal defects in an LD increase the threshold current density and then shorten the lifetime of the LD, and thus reduction of the defect density is important. There also exists a problem that light-emitting devices fabricated on a wafer produced by ELOG have respective emission outputs greatly different from each other depending upon their position on the wafer, because the dislocation density in the wafer is higher in some regions and lower in the other regions. Then, emission patterns were observed in light-emitting devices with emission outputs more than 2 mW and light-emitting devices with emission outputs less than 0.5 mV that were fabricated on the same wafer. It was found that the devices of lower outputs cause non-uniform radiation in which dark and bright portions were mixed. In addition, the lower-output devices had their shorter lifetimes and 90% thereof stopped emission shortly after electric current is supplied. Due to this, the total yield of the devices was as low as about 45%. The dark portions in the lower-output devices correspond to regions with high dislocation density in the GaN substrate, and it is considered that the defects in the GaN substrate affect the dark portions.
In view of the problems in the prior art discussed above, an object of the present invention is to improve the operating voltage, luminous efficiency, lifetime, and yield in the nitride-based semiconductor light-emitting devices.
A nitride-based semiconductor light-emitting device according to the present invention includes a semiconductor stacked-layer structure including a plurality of nitride-based semiconductor layers grown on a GaN-based substrate by vapor phase deposition. An interface region of the GaN-based substrate contacting the semiconductor stacked-layer structure contains oxygen atoms at a concentration n in the range of 2xc3x971016xe2x89xa6nxe2x89xa61022 cmxe2x88x923, and then the semiconductor stacked-layer structure has a lower crystal defect density as compared with that in the case that the interface region does not contain oxygen atoms at such a concentration n.
The GaN-based substrate may contain at least one of chlorine and oxygen. A nitride-based semiconductor layer included in the semiconductor stacked-layer structure, which is in direct contact with the GaN-based substrate, may contain oxygen.
With reference to FIG. 1, an explanation is here given regarding oxygen doping in the interface region of the GaN substrate that is in contact with the nitride-based semiconductor stacked-layer structure. FIG. 1 shows SIMS (secondary ion mass spectrometry) profiles obtained by oxygen doping in the vicinity of the interface between the GaN substrate and a nitride semiconductor layer grown thereon by MOCVD. In this graph, the horizontal axis represents layer thickness (nm) and the vertical axis represents concentration (cmxe2x88x923) of oxygen atoms. The layer thickness of 0 nm represents a surface when SIMS is started, and the oxygen atom concentration of 1016 cmxe2x88x923 corresponds to the concentration without positive or effective doping of oxygen atoms. Any ion concentration lower than 1016 cmxe2x88x923 is difficult to identify due to noise in SIMS.
The oxygen doping in the present invention is effective in relaxing strain caused in the interface region between the substrate and the crystal growth layer and preventing deterioration of crystallinity from being caused by N escape, Ga escape, etc. in the vicinity of the interface. In this case, the interface region of the substrate that contacts the crystal growth layer may have a thickness of single-atom layer to be doped with oxygen. However, the interface region is preferably doped in a thickness range that is likely to suffers damage during new crystal growth. Specifically, the advantage discussed above becomes clear when the interface region is doped in a thickness of at least 1 nm and becomes clearer when doped in 20 nm thickness. The interface region may be doped in a thickness exceeding 20 nm, but the doping effect with such a large thickness does not show much difference.
FIG. 1 shows SIMS profiles obtained by measuring oxygen distribution near the interfaces when oxygen atoms are added to the interface regions of at least 15 nm thickness in the substrates. SIMS measurement does not have a high accuracy with respect to the thickness direction and thus it is considered that oxygen would be observed in a range slightly greater than that of the region to which oxygen atoms are actually added.
The profile represented by curve A in FIG. 1 is obtained actually by adding oxygen atoms to a GaN buffer layer formed directly on a GaN substrate for fabricating a light-emitting device, and thus oxygen atoms are not directly added into the substrate. Regarding curve A, therefore, it is considered that the oxygen atoms diffuse into the formed substrate due to thermal hysteresis of heating during fabricating the light-emitting device on the substrate. Similarly, the profile represented by curve B in FIG. 1 is obtained by exposing a formed substrate to the atmosphere and thereafter forming a light-emitting device structure on the substrate, and thus oxygen atoms are not directly added into the substrate nor into the light-emitting device structure thereon. For curve B, however, oxygen atoms are detected in both of the substrate side and the light-emitting device side. Therefore, it is considered that oxygen atoms absorbed on the substrate surface in the atmosphere diffuse into both of the substrate and the light-emitting device structure due to thermal hysteresis of the substrate.
FIG. 2 shows the change of the emission output of a blue LED formed by HVPE on a GaN substrate, with respect to the oxygen doping amount in the interface region of the substrate. In this graph, the horizontal axis represents oxygen doping amount (cmxe2x88x923) in the interface region of the substrate and the vertical axis represents emission output P0 by an arbitrary unit (a.u.). The emission output observed when no oxygen is added to the interface region of the substrate is defined as reference value 1 of the arbitrary unit.
Referring to FIG. 2, the emission output is 1.2 (a.u.) when the oxygen doping amount is 2xc3x971016 cmxe2x88x923. As the doping amount increases therefrom, the emission output steeply increases. When the doping amount is about 1018 cmxe2x88x923, the emission output reaches the maximum value of 2.5, and thereafter the emission output gradually decreases. The emission output is still 1.1 even if the doping amount increases to 1022 cmxe2x88x923. It is thus seen that the emission output is enhanced by oxygen doping. However, when the doping amount is increased to or greater than 2xc3x971022 cmxe2x88x923, the emission output decreases to 0.6 or smaller. Possible reasons for these phenomena are described below.
Because of the high equilibrium vapor pressure of nitrogen as described above, nitrogen atoms escape out of the GaN substrate, particularly out of the region near the substrate interface, so that lots of vacancies of N sites are generated and accordingly the defect density increases. Then, a nitride semiconductor layer directly grown on this substrate by MOCVD etc. generates dislocation defects etc. that result from the strain of the substrate. Here, the substrate is doped with oxygen having a greater bonding force with Ga than that of nitrogen and thus being thermally stable, and the added oxygen atoms accordingly move into and fill the vacancies of N sites. Moreover, respective atomic radii of nitrogen and oxygen are almost equal to each other and thus strain in the substrate is unlikely to occur even if oxygen atoms fill N sites. Therefore, oxygen doping can reduce the crystal defect density. In addition, oxygen acts as a donor in the GaN crystal, which reduces the resistivity of the GaN crystal. However, an oxygen atom concentration equal to or lower than 1016 cmxe2x88x923 is not enough to fill all of the vacancies of N sites with oxygen atoms, so that reduction of the defect density, improvement of the emission output of a resultant light-emitting device, and decrease of the resistivity of the GaN crystal can not be achieved.
On the other hand, when oxygen atoms with a concentration equal to or higher than 2xc3x971022 cmxe2x88x923 are added into the substrate or the region near the substrate interface, the oxygen atoms replace nitrogen atoms and further become interstitial atoms to strain the GaN crystal. As a result, the dislocation defects in a resultant light-emitting device increases as the doping amount excessively increases to cause the emission output not to increase but to drastically decrease.
In a GaN substrate produced by HVPE, chlorine atoms exist in N sites or as interstitial atoms and there is the difference in atomic radius between chlorine and nitrogen, so that strain is caused in the GaN crystal. A nitride semiconductor layer directly grown on this substrate by MOCVD etc. thus includes dislocation defects etc. resultant from the strain in the substrate. Here, when oxygen atoms having high reactivity are added into the substrate or substrate interface, oxygen atoms instead of chlorine atoms predominantly enter N sites within the GaN crystal and act as donors. The difference in atomic radius between oxygen and nitrogen is smaller than that between chlorine and nitrogen. Thus, oxygen atoms can be added into the GaN substrate containing chlorine atoms to reduce dislocation defects etc. In this case too, however, the oxygen doping amount of 1016 cmxe2x88x923 or less cannot sufficiently fill vacancies of N sites with oxygen atoms and it is not enough to reduce the defects. On the other hand, when oxygen atoms are added at a concentration of 2xc3x971022 cmxe2x88x923 or higher, the increased oxygen atoms present as interstitial atoms strain the crystal. As a result, dislocation defects increase in a resultant light-emitting device and lower the emission output of the device.
For the reasons described above, the present invention can reduce the crystal defect density in a light-emitting device fabricated on a GaN substrate by doping a region near the GaN substrate interface with oxygen atoms at a predetermined concentration. In this way, nitride semiconductor light-emitting devices having a high luminous efficiency and a long lifetime can be produced with satisfactory yield. Moreover, the reduced crystal defect density decreases pass current, and the oxygen acts as a donor to decrease the resistivity of the substrate. Consequently, the drive voltage of the light-emitting devices can be reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.